In order to meet requirements of large-power equipment such as electric automobiles on voltage, power and energy during operation, a cell pack is generally formed of many batteries connected in series for use. Although the difference between cells decreases gradually with the improvement of technical processes, it is still difficult to guarantee the full consistence of characteristics of all cells in the level of the current manufacture processes. Particularly under operating conditions, irregular charging and discharging are frequently performed, the difference between cells will deteriorate after the cell pack works for a certain period of time, as a result, the utilization efficiency of the cell pack is reduced, and the service life of the cell pack is shortened.
The inconsistence between cells cannot be completely eliminated, particularly such inconsistence existing in the initial stage of manufacture. In order to guarantee the service life of a cell pack, a cell management system emerges as required, the main task of the cell management system is to detect and narrow the difference between cells, that is, the usually called cell equalization technology.
Cell equalization must start in a case that the individual voltage difference is far greater than the measurement error, but the effective equalization time will be too short when the error is great, to avoid this effect, equalization can be started only when cells are to be fully charged or completely emptied, resulting in quite poor equalization effect. Therefore, only equalization performed in a case that the voltage difference of individual battery cells is far greater than the measurement error makes sense, for example, if the measurement error of the individual battery cells is greater than the actual voltage difference between the individual battery cells, the measurement difference rather than the actual difference is equalized, in this case, cell equalization just deteriorates the cell pack instead of prolonging the service life of the cell pack.
Thus, it is crucial to detect the difference between all individual battery cells accurately and timely.
At present, methods for measuring a voltage difference of individual battery cells in a battery pack are almost as follows: measuring an absolute value of voltages at two ends of each individual battery cell, and then calculating the difference between the absolute values via software. For example:
Method 1: A voltage acquisition module is in turn connected to each individual battery cell via a high voltage switching network (often formed of relays), or the voltage of each individual battery cell is in turn transferred to a sampling capacitor, the sampling capacitor is then connected to a voltage measurement module to directly measure the voltage of the individual battery cell, and then the difference between the individual battery cells is calculated via software.
Since there are a large number of individual battery cells connected in series, the voltage of the high voltage resistant individual battery cells is superposed to a very high common-mode voltage, in combination with the voltage drop caused by the constantly changing working current to the internal resistance of the individual battery cells, the terminal voltage of each individual battery cell cannot be accurately measured in the prior art. Meanwhile, as the switching network is high in cost and low in speed, a passive switch is poor in durability, and an active switch is high in electric leakage and poor in reliability, the above method cannot be widely applied in an actual battery system.
Method 2: A battery pack formed of a large number of individual battery cells connected in series is divided into multiple modules, each module only comprising 4 to 16 individual battery cells connected in series. In this way, the total common-mode voltage in each module is limited, a voltage resistant (lower than 60V in general) differential amplifier may be adopted to remove the common-mode voltage to obtain the voltage value of each individual battery cell, the voltage value is then converted into a digital signal via an analog-to-digital converter (ADC) for timeshare inspection, and the digital signal is finally processed by a processor of a cell module.
In a practical environment with electromagnetic interference and also voltage drop caused by a dynamic current to internal resistance, a measurement result obtained by timeshare inspection has many different types of random interferences that cannot be thoroughly eliminated, an insufficiently inaccurate voltage measurement result of the individual battery cell may be obtained according to statistical methods through complex analysis on results of many times of measurement. In this way, the complexity of the system software is greatly improved, thus resulting in increased cost, decreased reliability and increased power consumption of the system, and reducing the technological and economic values of the whole management system. Meanwhile, the differential circuit has restrained actual effect due to element precision and parameter drift, and is not guaranteed to measure the voltage of the individual battery cell accurately for a long period of time.
In conclusion, the existing methods for collecting a difference between the individual battery cells almost employ conventional thoughts as follows: first, collecting absolute voltage values at two ends of each individual battery cell, and then calculating the difference between the individual battery cells based on the absolute voltage values. These methods have not only large error of the collected data but also high cost for collecting the data, and are disadvantageous for the cell management system to perform equalization management to the individual battery cells.